Due to recent energy problems such as possible exhaustion petroleum resources, etc., efficient fuel cells have been now brought into the limelight. The fuel cell is a system of generating electricity by reaction of hydrogen with oxygen, where how to store hydrogen is a problem. Up to now, how to store hydrogen under high-pressure, how to store hydrogen by adsorption onto a metal, how to reform hydrocarbon to recover hydrogen, etc. have been proposed. Storage of hydrogen gas under high-pressure must take a tank storage form.
Now, tanks capable of withstanding a hydrogen gas storage pressure of about 35 MPa are generally used, but cannot satisfy a sufficient mileage when the fuel cells are to be mounted on automobiles. Thus, storage under much higher-pressure, for example, about 70 MPa, has been now under study. Seals are indispensable for storage of hydrogen gas in a tank, and now metallic seals, EPDM rubber seals, etc. have been proposed, but in the present situation the metallic seals have a poor maintainability, whereas the EPDM rubber seals have a poor reliability.
Functions generally required for a high-pressure gas seal are an unbreakability due to pressure increases or pressure reduction (no occurrence of blisters) and a maintainability of rubbery elasticity even if exposed to extremely low temperatures by adiabatic expansion due to an abrupt pressure reduction. For example, it is known that CNG (compressed natural gas: about 20 MPa) known as a high-pressure fuel gas undergoes a temperature decrease down to about −60° C. by adiabatic expansion when subjected to abrupt pressure reduction. In view of this fact, it seems necessary that the rubbery elasticity must be maintained even at −60° C. or lower, because the hydrogen gas is stored under higher pressure than that of CNG. However, EPDM as now mainly used can maintain a satisfactory rubbery elasticity down to about −50° C., and thus has unsatisfactory low-temperature characteristics.
Seal materials in a tight seal structure capable of sustaining tight sealing of high-pressure gases such as high-pressure hydrogen gas, etc., so far used, include, for example, butyl rubber, fluororubber, hydrogenated nitrile rubber, tetrafluoroethylene-perfluoro(alkyl vinyl ether) copolymer, etc., as disclosed in the following Patent Literatures 1 and 2. However, these rubber materials for forming seal materials are not preferable, because of deterioration of sealability at low temperatures. Furthermore, at least butyl rubber, fluororubber, and hydrogenated nitrile rubber have a problem of blister occurrence, when subjected to abrupt pressure reduction.    Patent Literature 1: JP-A-2003-28302    Patent Literature 2: JP-A-2004-76870
Silicone rubber is also known as rubber having distinguished low-temperature characteristics, where polydimethylsiloxane having a small amount of vinyl groups as cross-linkable groups, most typical silicone rubber, has a low-temperature resistance down to about −55° C., and thus has no satisfactory low-temperature characteristics, whereas methylfluoroalkylvinyl-based silicone rubber has an effective low-temperature resistance down to about −70° C., but has a problem of blister occurrence when subjected to abrupt pressure reduction.
The present applicants have so far proposed a silicone rubber composition, which comprises: (a) diorganopolysiloxane copolymer comprising (i) about 5 to about 40% by mole of (CF3CH2CH2)a(R)bSiOc and (ii) about 95 to about 60% by mole of R(a+b)SiOc as unit formulae, where R is methyl, ethyl, vinyl or phenyl group, (b) diatomaceous earth, (c) finely powdery silica, (d) titanium oxide and carbonate or oxide of Mn, Zn, Mg, Ca or Fe, (e) graphite, (f) fluororesin, (g) carbon black, and (h) an organic peroxide, which is disclosed to have distinguished heat resistance and oil resistance, and to be applicable effectively to mold seal materials.    Patent Literature 3: JP No. 2,049,956
It is further disclosed therein that in the afore-mentioned unit (i), a (CF3CH2CH2)(CH3)SiO unit and a (CF3CH2CH2)(C2H5)SiO unit can be used as preferable copolymerization units, and in the unit (ii), a (CF3)2SiO unit can be used so, and the copolymer molecule chain terminals may be blocked by a triorganosilyl group, etc. such as a trimethylsilyl group, a dimethylvinylsilyl group, a methylvinylphenylsilyl group, etc. It is stated that vulcanization-molding products of the composition can be effectively applied to various packings, sealing materials, etc. in the field relating to machines and transporting machinery, but no mention has been made therein as to application to high-pressure gas sealing materials requiring good low-temperature characteristics and blister resistance.